Glow discharge tube



July 23, 1957 FIG] UNREGULATED VOLTAGE SUPPLY J. E. DRENNAN T AL GLOW DISCHARGE TUBE Filed Sept. 14. 1953 bypLTAcE INVENTORS, JAMES E. mam/mm FRANCIS 0. row.

AZTORNEX United States Patent GLQW DISCHARGE TUBE James E. Drennan, Columbus, and Francis C. Todd, Amlin, Ohio, assignors to the United States of America as represented by the Secretary of the Army Application September 14, 1953, Serial No. 380,148

Claims. (Cl. 313-193) This invention relates to voltage regulators and more particularly to regulators of the gaseous glow discharge type.

In prior voltage regulator devices utilizing glow discharge tubes, operation at higher currents results in oscillations in the output voltage due to the very irregular voltage current characteristics of the tubes.

Reference is made to the copending application of Francis C. Todd et 211., Serial No. 258,934, filed November 29, 1951, in which a so-called hollow cathode type of gaseous voltage regulator tube is disclosed and claimed. Such tubes have been operated successfully at currents as high as 800 milliamperes, but the best results are produced when operated over a current range of 10 to 460 rnilliamperes. However, when such tubes are operated in the upper regions of this current range, oscillations with amplitudes as high as volts peak appear in the output voltage which are similar to the oscillations which appear when conventional glow discharge tubes are operated at the upper end of their current range.

An object of this invention is to provide an electron discharge device having improved characteristics for reducing oscillations in the regulated output voltage.

Another object of this invention is to provide an improved hollow cathode voltage regulator tube having improved voltage regulating characteristics.

In accordance with this invention, the glow discharge device comprises a suitable attenuated gaseous atmosphere including one or more gases, such as neon and argon, and a trace of an impurity having a lower ionization potential than the metastable energy level of the other gases, such as iodine vapor, for reducing oscillations in the output regulated voltage.

For a better understanding of the present invention together with other and further objects thereof, reference is had to the following descriptions taken in connection with the accompanying drawing, in which:

Fig. 1 is a plan view, taken in section along BB of Fig. 2, of a gaseous discharge tube built in accordance with the invention;

Fig. 2 is an elevational view taken in section along A--A of Fig. 1; and

Fig. 3 is a schematic diagram of a voltage regulator circuit incorporating the novel gaseous discharge tube illustrated in Figs. 1 and 2.

.The hollow cathode referred to in this application is a type of cathode capable of maintaining a so-called hollow cathode glow. In order to produce a hollow cathode glow, it is necessary to have in the envelope of a glow discharge tube two or more cathode elements spaced not more than two cathode dark spaces from each other; otherwise the hollow cathode glow will not form. Such a hollow-cathode glow is produced by the interaction of two cathode glows, in the interspace between the cathode elements and not merely between the anode and the nearest cathode element.

.The hollow cathode glow tube is to be distinguished from the anode glow, which exists between the anode and the extreme edges of the cathode elements, whereas the hollow cathode glow exists in the interspace between the cathode elements. In a tube using a hollow cathode, the discharge strikes from the anode to the spaced edges of the cathode elements. A very intense illumination appears between the spaced elements and, at the upper end of the current range, this new illumination extends between the spaced cathode elements from their top to bottom and outwardly toward the sides. In addition to the new illumination which appears between the spaced cathode elements, the anode glow, which is due to an anode fall of potential, is also present. The spaced cathode elements usually have a field free space therebetween, which constitutes the interspace referred to above. The extent of the interspace is determined by the pressure of the filling gas.

It has been found that the oscillations in the regulated output voltage are associated with the anode glow. The conditions for the appearance of the anode glow are well known and have been widely discussed in the literature. According to the generally accepted theory of Langmuir and Mott-Smith, which appeared in General Electnc Review 27, 762 (1924), a positive anode fall develops when:

jr= A Hel where n is the number of ion pairs per cubic centimeter, e is the electron charge, and 1 is the average velocity. Thus, a positive anode fall develops when:

i net F 3 The anode glow appears when the anode fall of potential has increased until it approaches the ionization or resonant potential of the gas.

From Equation 3, it can be seen that the anode fall of potential can be eliminated by making the right-hand side of Equation 3 as large as possible. The anode fall of potential cannot appear when the right-hand side of Equation 3 exceeds the total tube current. One way of reducing the anode fall of potential is to use one or more screen grids between the anode and cathode, as described and claimed in the application of Elmer C. Lusk et 211., Serial No. 380,149, filed on even date herewith. In accordance with the present invention, this is accomplished by increasing the random current density jr, which is brought about by increasing n, the number of ion pairs per cubic centimeter. This increase is obtained by the addition of a relatively minute amount of impurity to the gaseous atmosphere. For example, in the case of a neon discharge, a small amount of argon, usually less than 1%, is added, though less than 0.1% has been found to be the optimum amount.

Referring now to Figs. 1 and 2, there is shown a voltage regulator tube 19 composed of an anode 18, wire mesh screens 20 and 22, and a hollow cathode composed of a pair of cathode elements 12 and 14, all enclosed within a sealed envelope 16. Although all these elements are shown as concentrically disposed cylinders, it is to be understood that the invention is not restricted to such construction. Cathode elements 12 and 14 may be connected together internally or externally so as to maintain them at the same potential. The tube leads to the various elements and their supports have been omitted for the purpose of clarity. The envelope 16 is filled with the usual attenuated gaseous atmosphere suitable for maintaining a glow discharge comprising at least one of the following gases: hydrogen, nitrogen, helium, neon, krypton, argon, xenon, mercury vapor and sodium vapor. In accordance with this invention, a trace of an impurity such as cesium vapor, gallium vapor, or iodine vapor is added to this atmosphere.

The constructional details of the tube elements depend, of course, on the desired characteristics. In one tube built, the details are as follows: anode 18 may be made of nickel, and is shaped in the form of a solid cylinder having a diameter of approximately Screens 20 and 22 may be made of nickel or tungsten. These screens surround the anode for a portion of its length. The first screen 20 is spaced approximately A from the anode and has a diameterof and encloses as much of the anode as possible. The second screen 22 is spaced approximately from the first screen and has a diameter of A4" and totally encloses screen 20. Cathode elements 12 and 14 are preferably made of molybdenum or tungsten, although other metals are suitable. Cathode element 14 is 1% in diameter and is spaced A." from anode 18. The space between cathode elements 12 and 14 is 0.020" to 0.060, the exact spacing being dependent upon the gas pressure, which, as above pointed out determines the extent of the dark space. The screens consist of a grid made up of 0.032 x 0.125" nickel strips with 0.125 spacing therebetween.

The circuit shown in Fig. 3 employs the voltage regulator tube with an unregulated voltage supply 24 to produce a regulated output voltage at 26. The anode 18 is connected to the positive terminal of the voltage supply 26 through resistor 28 and cathode elements 12 and 14 of the hollow cathode are connected to the negative terminal of the voltage supply. The screens and 22 are connected respectively through resistors 30 and 32 through resistor 28 to the positive terminal. The current to the screens is controlled by the size of resistors 30 and 32. Variation of these resistors will vary the anode current. The regulated output voltage at 26 is taken across anode 18 and the common junction of cathode elements 12 and 14, and will be free of any large voltage oscillations. Cathode elements 12 and 14 may be connected internally or externally.

As was mentioned previously, the anode fall of potential can also be eliminated by increasing the ionization coefficient. The ionization coefficient is greatly increased when a small quantity of argon is added to a neon discharge. This increase in ionization arises because the metastable energy level of neon is slightly greater than the ionization potential of argon. When a neon atom in the metastable state collides with an argon atom in the ground state, the argon atom is ionized.

The probability of ionization is approximately one, for the optimum, small values of the voltage gradient.

For example, a hollow cathode glow discharge tube using molybdenum for the hollow cathode and filled with very pure argon gas had a regulation of approximately 7 volts for a current range from 20 to 460 milliamperes, while a similar tube filled with pure neon gas showed a regulation of 4 volts for the same current range. However, oscillations with amplitudes as high as 20 volts peak were produced.

It has been found that with the use of screens and with 0.1% argon in neon, the oscillation amplitude is appreciably decreased to 0.3 volt, whereas the optimum amount of argon in neon was found to be between 0.02% and 0.075% with a reduction of the oscillation amplitude to 0.0035 volt, for a current range from 20 to 400 milliamperes. However, since it is difficult to vary the percentage of argon in neon, slightly larger oscillation amplitudes are obtained, the optimum conditions being quite diflicult to obtain.

Since it is dificult to vary the percentage of argon added to the neon in a tube, it is proposed, in accordance with this invention, that another noble gas or other suitable gas, such as a trace of iodine vapor, be added. It has been found that the optimum amount of argon to be added to neon need be less than 1% when a trace of iodine vapor is also added to the argon-neon mixture. The amplitude of the oscillations produced with the aforesaid gas mixture was reduced to 0.0020 volt, which is considerably lower than the 0.0035 volt oscillation amplitude produced with the addition of any percentage of argon alone.

The trace of iodine added may vary between the limits of 0.01% to 0.1%, with the percentage much closer to 0.01%. The exact amount of the trace of iodine which can be inserted into the tube is determined by the temperature and pressure used. The temperature and pressure determining the exact portion, which is usually about 0.01 percent, though it can approach 0.1 percent of the total gas mixture under certain conditions.

With the addition of the trace of iodine vapor to the neon-argon mixture, the percentage of argon need only be some percentage less than 1% in order to obtain optimum conditions with the consequent reduction of the oscillation amplitude to approximately less than 0.0020 volt. The results obtained with the addition of a trace of iodine vapor to the neon-argon mixture shows that the density of ionization may be disproportionately increased by the addition of another gas having an ionization potential which is less than the lowest metastable state of the gases in the tube, and that the additive gas may be a noble gas or any other gas which does not have undesirable effects on the other characteristics of the tube. The general principle is to add a gas with an ionization potential which is lower than a populated metastable state of the gases already in the discharge.

It has also been found that, the ionization coefficient in a glow discharge tube filled with helium gas can be increased by the addition of a small percentage of argon, krypton, or xenon in much the same way that argon increased the ionization coefiicient in the neon-filled voltage regulator tube. Also, the ionization coefficient in an argon-filled tube could be increased by the addition of a trace of cesium or gallium vapor in addition to or in place of iodine vapor.

Temperature considerations are not too important over an ordinary range, but might become important below -l00 C. However, it is not usually required that the tube operate at such a low temperature. The pressure of the gas already within the envelope determines the optimum amount of the additive gas which is to be added. The pressure of the gas in the tube and the spacing of the hollow cathode elements are interdependent.

Many of the gases mentioned previously which can be used as a filling gas for the voltage regulator tubemay react in varying degrees with each other, and with the iodine vapor. Only those gases can be added to each other which will not produce deleterious effects upon the tube elements or react with each other or the iodine vapor. It has been found that the iodine vapor will react with almost all of the materials of which the tube elements can be constructed. However, molybdenum and tungsten are least affected by iodine vapor.

The gases to be added to each other are related by their ionization potential. In order to produce the desired results, the gas to be added to the gases already in the envelope 10 should have an ionization potential which is lower than the populated metastable state of the gases already within envelope 10. For example, argon may be added to helium but not to krypton or xenon. Iodine may be added to any one of the noble gases, and to any mixture of noble gases. When argon is added to neon, other gases can be added having an ionization potential which is lower than the metastable level of argon, which is about 11.49 electron-volts. Also, the ionization coeflicient in an argon-filled tube could be increased by the addition of a gas such as iodine, cesium, or gallium. Some of the satisfactory gases which can be added to the neonargon mixture are iodine, which has an ionization potential of 10.6 electron-volts, mercury with an ionization potential of 10.39 electron-volts, gallium with an ionization potential of 5.97 electron-volts, and cesium with an ionization potential of 3.87 electron-volts. If mercury vapor is used as the main gas, only cesium vapor can be employed, since the metastable energy level of mercury vapor is 4.9 electron-volts and cesium, which has an ionization potential of 3.87 electron-Volts, is the only gas having an ionization potential which is lower than the energy in the metastable state and is closest to this energy.

The oscillation amplitude in the output of a hollow cathode tube is dependent upon the pressure of the gases in the tube and the space between the cathode elements 12 and 14. For example, in a hollow cathode tube constructed with screens and inwhich the cathode elements are spaced 0.040" and are made of molybdenum, and in which the main filling gas is neon, the oscillation amplitude voltage is 0.3 volt when the pressure is 100.8 millimeters of mercury, and is approximately 1 volt when the gas pressure is 91.5 millimeters of mercury. When 0.1% argon is added to a tube having the above structure, at a gas pressure of 100.8 millimeters of mercury, the oscillation amplitude voltage is reduced to 0.1 volt, while if 0.5% argon were added at the same pressure the oscillation amplitude voltage is reduced to only 0.015 volt. A hollow cathode tube having the same structure as above but with a gas pressure of 93.6 millimeters of mercury and 0.1% argon as the additive gas, will produce an oscillation amplitude voltage of 0.0035 volt; and, if a trace of iodine is now added to this tube the oscillation amplitude voltage is reduced to 0.0020 volt. In a tube having the same structure as the above tube, but with neon gas at a pressure of 125 millimeters of mercury, the oscillation amplitude voltage is volts. If 0.1% argon is added, the oscillation amplitude voltage is reduced to 0.15 volt, whereas if only 0.05% is added, the oscillation am plitude voltage is reduced to 0.0030 volt. From the above figures, it can be readily seen that the gas pressure and the spacing of the cathode elements are interrelated and determine the oscillation amplitude; however, when a trace of an impurity, such as iodine, is added to a neonargon mixture, the oscillation amplitude voltage is further reduced.

From the above examples, it can be seen that the oscillation amplitude can be reduced by using screens alone proximate to the anode, but a further reduction is obtained by adding a trace of iodine. Other impurities, such as cesium or gallium, may also be used, but the addition of a trace of these impurities alone, i. e., without the use of screens, will also reduce the oscillation ampli- Itjuc'tllel, although to a lesser extent than the combination of The invention is not limited to hollow cathode voltage regulator tubes but can also be used with conventional cathode voltage regulator tubes where high output cur rents are desired.

Since many widely dilferent embodiments of the invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments disclosed.

What is claimed is:

1. A glow discharge tube comprising an envelope having therein an anode, a hollow cathode, and an attenuated gaseous atmosphere, said cathode having spaced surface portions defining therebetween a gaseous space having an extent which is not greater than twice the extent of the cathode dark space, said atmosphere consisting essentially of one or more of a group of gases having at least a given metastable energy level, and less than 1% of an additional gas having an ionization potential which is than said metastable energy level, said gases being substantially non-reactive with each other.

2. A glow discharge tube as set forth in claim 1, wherein said group of gases consists of hydrogen, nitrogen, helium, neon, krypton, and xenon.

3. A glow discharge tube as set forth in claim 2, wherein said additional gas is argon.

4. A glow discharge tube as set forth in claim 3, including a trace of one or more gases of the group consisting of iodine, cesium, and gallium.

5. A glow discharge tube as set forth in claim 1, wherein said atmosphere consists essentially of neon and less than 1% of argon.

6. A glow discharge tube as set forth in claim 5, where in the amount of said argon is between 0.02% and 0.075%.

7. A glow discharge tube as set forth in claim 5, wherein said atmosphere includes in addition a trace of iodine.

8. A glow discharge tube as set forth in claim 7, wherein the amount of said iodine is between 0.01% and 0.1% of the entire mixture.

9. A glow discharge tube as set forth in claim 1, wherein said atmosphere consists essentially of neon, less than 1% of argon, and between 0.01% and 0.1% of iodine, and wherein said anode is cylindrical and wherein said cathode comprises a pair of cylinders concentric with said anode.

10. A glow discharge tube as set forth in claim 9, including a cylindrical grid between said anode and cathode and coaxial therewith.

11. A glow discharge tube as set forth in claim 1, wherein said anode is cylindrical, and wherein said cath ode comprises a pair of cylinders coaxial with said anode.

12. A glow discharge tube as set forth in claim 11, including a cylindrical grid between said anode and cathode and coaxial therewith.

13. A glow discharge tube as set forth in claim 1, including a trace of an additional gas having a metastable energy level which is less than that of the other gases.

14. A glow discharge tube as set forth in claim 13, wherein said additional gas is one of the group consisting of cesium, gallium, and iodine.

15. A glow discharge tube as set forth in claim 13, wherein said additional gas is iodine the amount of which is between 0.01% and 0.1% of the entire mixture.

References Cited in the file of this patent UNITED STATES PATENTS 2,469,410 Rentschler May 10, 1949 2,473,642 Found et al June 21, 1949 2,487,437 Goldstein et a1. Nov. 8, 1949 2,586,836 Liebson Feb. 25, 1952 2,647,217 Foulke July 28, 1953 

1. A GLOW DISCHARGE TUBE COMPRISING AN ENVELOPE HAVING THEREIN AN ANODE, A HOLLOW CATHODE, AND AN ATTENUATED GASEOUS ATMOSPHERE, SAID CATHODE HAVING SPACED SURFACE PORTIONS DEFINING THEREBETWEEN A GASEOUS SPACE HAVING AN EXTENT WHICH IS NOT GREATER THAN TWICE THE EXTENT OF THE CATHODE DARK SPACE, SAID ATMOSPHERE CONSISTING ESSENTIALLY OF ONE OR MORE OF A GROUP OF GASES HAVING AT LEAST A GIVEN METASTABLE ENERGY LEVEL, AND LESS THAN 1% OF AN ADDITIONAL GAS HAVING AN IONIZATION POTENTIAL WHICH IS LESS THAN SAID METASTABLE ENERGY LEVEL, SAID GASES BEING SUBSTANTIALLY NON-REACTIVE WITH EACH OTHER. 